1. Field of the Invention
This invention relates to a thermal head and more particularly to a corner head type thermal head improved in printing efficiency and a manufacturing method therefor.
2. Description of the Related Art
In using a thermal head, It is necessary to concentrate pressure on a ribbon, print paper, and a platen in a heater area for supporting print on rough paper and improving printing efficiency.
For this purpose, hitherto, a near edge type thermal head with a heater area provided near the edge of the thermal head has been installed so as to be inclined against a platen for concentrating pressure against a ribbon and print paper on the heater area and its vicinity. Such a near edge type thermal head is disclosed, for example, in Japanese Utility Model Publication No. Hei 4-46929. An example of the head is shown in FIGS. 30 and 31.
FIG. 30 shows a sectional view of the near edge type thermal head, wherein a glaze layer is formed on a substrate 10 and a resistance film layer 12, electrodes 14 and 15, and a protective film are provided thereon. Substrate 10 may be made of any material suitable for its purpose.
To use the thermal head, print paper 30 is placed under a platen 51 and from under the print paper, a ribbon 31 is pressed onto the print paper 30 by the thermal head 50, as shown in FIG. 31. At this time, the thermal head 50 is supported by a carriage so that it is inclined against the platen 51. As shown in FIG. 30, a slope 18 constituted by a part of the glaze layer 11 and a part of the substrate 10 is formed on the edge of the thermal head 50 for facilitating passage of the ribbon 31 when the thermal head 50 is inclined. However, the intersecting part 55 of the slope 18 and the top surface of the glaze layer 11, namely, the corner part 55 is not applied on a heater area 13.
The near edge type thermal head has a heater part having a small curvature and the heater area is formed so as not to lie across the corner part 55, thus the inclination angle of the thermal head against the platen 51 cannot be made large. The angle is from several degrees to less than 10 degrees at most. Therefore, concentration of pressure on the ribbon and print paper cannot be made so high with the result that printing efficiency is insufficient and a good print on rough paper cannot be provided.
To solve the problem, a corner head type thermal head is used with the above-mentioned corner part formed in a glaze layer located near the edge of the thermal head and a heater area formed so as to lie across the corner part. Examples of such a corner head type thermal head are shown in FIGS. 32 and 33(a),(b).
FIG. 32 shows a sectional view of an example of the conventional corner head type thermal head, wherein a glaze layer 11 is formed on a substrate 10 and a resistance film layer 12 is formed on the glaze layer 11. The glaze layer 11 in the example is of partial glaze type and has the sectional form like a mountain. A heater area 13 with a predetermined part generating heat when the print operation is performed is formed on the top of the mountain. A slope 18 is provided from the heater area 13 to the side 17 of the substrate 10 edge near the heater area 13. A common electrode 15 is provided on the slope 18. A discrete electrode 14 for supplying a current to the predetermined part of the heater area 13 in conjunction with the common electrode 15 is formed in an area on the resistance film layer 12, the area facing the common electrode 15 with the heater area 13 between. In the example, current flows from the common electrode 15 via the resistance film layer 12 of the heater area 13 into the discrete electrode 14.
As seen in FIG. 32, the slope 18 ends on the side of the glaze layer 11, namely, the corner part 55 is formed so as to be applied on the heater area 13. Therefore, the heater area 13 is formed so as to lie across the corner part 55.
A protective film 16 is formed on the top layer.
FIGS. 33(a),(b) show another example of the conventional corner head type thermal head. Parts identical with or similar to those previously described with reference to FIG. 32 are denoted by the same reference numerals in FIGS. 33(a),(b) and will not be discussed again.
FIG. 33 (a) shows a sectional view of the thermal head in the example. A discrete electrode 14 and a common electrode 15 are provided on the same side with respect to a heater area 13 and a turned common electrode 45 is provided facing the discrete electrode 14 and the common electrode 15 with the heater area 13 between.
Their arrangement is shown as a partial plan view in FIG. 33 (b), wherein supply voltage is supplied to the common electrodes 15 and current flows into the discrete electrodes 14 via the heater area 13 and the turned common electrodes 45.
Next, FIG. 34 shows a use example of the conventional corner head type thermal head.
In FIG. 34, the corner head type thermal head 50 is installed so as to be inclined against a platen 51 for concentrating pressure against a ribbon 31 and print paper 30 on the heater area 13, in the example, the inclination angle of the corner head type thermal head can be made larger than that of the near end type thermal head; normally, it can be set to about 10 degrees to 35 degrees. The curvature of the heater part of the corner head type thermal head can also be made larger than that of the near end type thermal head. Thus, the concentration of pressure is raised, improving the printing efficiency.
However, since the slope 18 is flat, the intersecting part 20 of the side of the thermal head and the slope 18 has a corner. The curvature of the heater part becomes large, the head sinks into the ribbon 31 and print paper 30 deeply, and the inclination angle increases, so that the intersecting part 20 approaches the ribbon 31, etc., compared with the near edge type thermal head.
Thus, the ribbon 31 is in sliding contact with the top of the intersecting part 20 and is worn or cut. Dirty print occurs on print paper 30 because of powder from the ribbon 31.
Further, if thermosensible paper is used as print paper 30, it is also in sliding contact with the top of the intersecting part 20, causing pressure rubbing of the paper, so that it causes a mark.
In the example of the conventional corner head type thermal head shown in FIG. 32, the width of the slope 18, L, is about 200 .mu.m. Therefore, the width of the common electrode 15 formed in the part is limited to 200 .mu.m or less. If the common electrode 15 is made thicker, a disadvantage such as catching of the ribbon occurs and the thickness is also limited. Thus, if the heater area is lengthened or the number of heaters is increased in the conventional corner head type thermal head, the resistance value of the common electrode 15 becomes large and the voltage drop at the parts far from the part to which supply voltage is supplied becomes large, degrading the printing quality.
On the other hand, in the example of the conventional corner head type thermal head shown in FIG. 33, a power supply is connected to each of the common electrodes 15 individually, thus the voltage drop can be reduced and the problem in the example in FIG. 32 can be dealt with.
However, in the example shown in FIG. 33, the substantial area of the heater area 13 corresponding to one picture element becomes twice that in the example shown in FIG. 32; the corner head type thermal head in the example shown in FIG. 33 is not applicable to an application where a fine pattern is required.